1. Field of the Invention
This invention relates to an audio-signal-processing apparatus capable of harmonic-series-generation and a method thereof.
2. Description of the Related Art
Audio-signal-processing apparatuses with harmonic-series-generation have been proposed for various purposes. Generally known is, for example, an effector apparatus that gives variations to tone color of a musical instrument and voice, especially, a virtual reproduction system of bass sound that generates a harmonic-tone for a bass component to reinforce the bass sounds in a small speaker.
A prior art virtual reproduction system of bass sound will be described with reference to FIG. 9 through FIG. 13. FIGS. 9 (a) and (b) are block diagrams illustrating a conventional audio-signal-processing apparatus. Each harmonic-tone can be generated with a full-wave-rectification method, a power method, a zerocrossing method or other similar method.
A first example of a prior art system will be described with reference to FIG. 9 (a). In this example, a plurality of harmonic-tone components of integer orders are generated with the zerocrossing method or the power method for low frequency components.
As illustrated in FIG. 9 (a), a signal inputted from an input terminal 91 is divided into two systems. In the first system, all the components of the input signal are inputted into a delay unit 93, and the output of the delay unit is then inputted into one input of an adder 100 after undergoing gain adjustment by a gain-control 94a. The components of the input signal are delayed by a time that is required for processing in a harmonic-series-generating unit 97, whose processing will be described later.
In the second system, all the components of the input signal are inputted into a low pass filter 96. The low pass filter 96 extracts only low frequency components from all these components according to a predetermined cut-off characteristic of the low pass filter 96, and outputs the extracted low frequency components to harmonic-series-generating unit 97.
Throughout this specification, a harmonic-tone with n times the frequency (n is a natural number) of the fundamental tone frequency (fundamental frequency) is referred to as an n-th harmonic-tone.
The harmonic-series-generating unit 97 includes harmonic-series-generating units 98a, 98b, . . . , 98c that generate a 2nd harmonic-tone, a 3rd harmonic-tone, . . . , an n-th harmonic-tone for the low frequency components that the low pass filter 96 has extracted. The 2nd harmonic-tone and the 3rd harmonic-tone, . . . , the n-th harmonic-tone that these harmonic-series-generating units 98a, 98b, . . . , 98c generate are inputted into another input of the adder 100 after undergoing gain adjustment by gain controls 94b, 94c, . . . , 94d respectively and then added in an adder 95.
The adder 100 adds the components inputted from the first system and the second system respectively and outputs the sum to an output terminal 92.
Another example of a prior art system will be described with reference to FIG. 9 (b). In this example, a harmonic-tone is generated for the low frequency component with the full-wave-rectification method. Japanese Patent Publication No. 05-328481 discloses a circuit pertaining to the second example.
In FIG. 9 (b), the explanation is omitted for the same components described in and with reference to FIG. 9 (a).
In FIG. 9 (b), the harmonic-series-generating unit 97 illustrated in the FIG. 9 (a) is replaced with a full wave rectifier 99 and a band pass filter 101.
The full wave rectifier 99 changes negative values of the low frequency signals that a low pass filter 96 has extracted to positive values, doubling the frequency of the low frequency signals. However, since direct-current bias and even harmonic-tone components are actually generated, only the 2nd harmonic-tone that is a principal component, is extracted using the band pass filter 101.
As illustrated in FIG. 9 (b), full-wave-rectification can be executed with a simple structure. The full-wave-rectification system can generate a second harmonic-tone and be expanded to generate harmonic-series on the order of an n-th power of 2 (n=2, 3, . . . ) by cascading plural units of the full-wave-rectification 99 and using a band pass filter 101 as illustrated in FIG. 9 (c). However, this full-wave-rectification system poses a problem in that odd harmonic-tones cannot be generated.
Next, problems associated with the zerocrossing method and the first example in the prior art will be described with reference to FIG. 10. FIGS. 10 (a) and (b) illustrate graphs exemplifying waveforms resulting from the prior art zerocrossing method the prior art. In the figures, the horizontal axis illustrates time and the vertical axis indicates amplitude.
The zerocrossing method detects the zerocrossing points P1, P2, and P3 as the signal changes from positive to negative or from negative to positive. These detected zerocrossing points are illustrated by the circles depicted in FIG. 10 (a).
In generating the second harmonic-tone component, a signal is double-compressed in the time axis and repeatedly reproduced twice in an interval between one zerocrossing point and the next zerocrossing point (for example, an interval between P1 and P2 or an interval between P2 and P3, in FIG. 10 (a)).
Similarly, an n-th order harmonic-tone is obtained by compressing a signal n times and repeatedly reproducing the signal n times in an interval between two consecutive zerocrossing points on the time axis.
A general music source is a complex tone that includes a plurality of pure tone components. For example, a periodical musical sound is includes a fundamental tone with (f=40 Hz in FIG. 11 (a)) and harmonic-tones with frequencies of integral multiples of the fundamental frequency (80, 120 . . . Hz), as illustrated in spectrum structure examples in FIG. 11 (a).
Further, a chord possesses a plurality of fundamental frequencies with strong energy components. For example, the chord possesses the spectrum structure illustrated in FIG. 11 (b) in Perfect 8th, and the spectrum structure illustrated in FIG. 11 (c) in Perfect 5th.
Next, problems encountered when using conventional harmonic-series-generating methods for such general complex tones (including a plurality of pure tones) will be described. Hereafter, a zerocrossing method used as a harmonic-series-generating method will be described. Additionally, problems encountered when using other harmonic-series-generating methods, such as the full-wave-rectification method and the power methods, will be described.
FIG. 12 (a) illustrates the results of the frequency analysis performed on the processed sounds, after generating the 2nd harmonic-tone at the same level as the original signal, by the zerocrossing method for complex tones (refer to FIG. 11 (b) regarding its spectrum structure). The complex tones include two pure tones (pure tones of 40 Hz and 80 Hz of the same level) that are in the Perfect 8th relation (the frequency ratio is 1:2).
FIG. 12 (b) illustrates the results of the frequency analysis performed on the processed sounds, after generating the 2nd harmonic-tone at the same level as the original signal, by the zerocrossing method for complex tones (refer to FIG. 11 (c) regarding its spectrum structure). The complex tones include two pure tones (pure tone of 60 Hz and 90 Hz of the same level) that are in the Perfect 5th relation (the frequency ratio is 2:3).
It is desirable for the processed sounds to possess only the second harmonic components of each pure tone component in both FIGS. 12 (a) and (b). However, although the second harmonic components are generated in either case, some distortions occurs due to the inclusion of other components than the second harmonic components (e.g., the fundamental frequency equal to the highest common factor of the fundamental frequency of two pure tone components). This distortion occurs since the harmonic-tone is generated at once for the signal with a plurality of pure tones.
As illustrated in FIG. 13 (a), in a signal with a plurality of pure tones, the waveform cycle T of the complex tone (the least common multiple of the contained pure tone cycles) and the interval of the zerocrossing points in the original waveform do not usually agree with each other.
For such a case, applying the zerocrossing method compresses the waveform and makes reproduction repeatedly in other interval than the zerocrossing points with cycle T, as illustrated in FIG. 13 (c). As a result, the processed signal waveform is not similar to the original signal and waveform distortion occurs. Comparing FIG. 13 (b) with FIG. 13 (c), FIG. 13 (b) illustrates the waveform that has the ideal harmonic-series-generation of the original waveform of FIG. 13 (a). Therefore, there is a need to overcome or lessen the disadvantages noted above in the prior art.